Contaminants may exist in subsurface soil in the liquid or vapor phase as discrete substances and mixed with and/or dissolved in ground water and soil gases. Such contaminants may be found and dealt with in accordance with this invention in the vadose (unsaturated) zone found between the surface of the earth and the water table, at the interface between the vadose zone and the water table, and in the saturated zone below the water table.
At many industrial and commercial facilities and at waste handling and disposal sites, soil and ground water are contaminated with suspended or water-soluble chemicals, or both. A variety of techniques have been used for removal of soil contaminants and remediation of affected soil. One common technique involves the excavation and off-site treatment of the soil. Another technique involves saturating the contaminated soil with water in situ, causing the contaminants to be slowly leached from the soil by the water. The contaminated water can then be removed.
Techniques have also been proposed for removing volatile organic contaminants from soil by vacuum extraction. For example, in U.S. Pat. No. 4,323,122, it was proposed that a vacuum be applied in a borehole at the level of the water table, the assumption being that a contaminant such as gasoline, which is lighter than water for example, would float on the water table and present a layer which could be drawn off by vacuum applied to the liquid at or around that level.
Others have suggested the possibility of venting soil above the water table (i.e., in the vadose zone) to cause vaporization of the contaminant in the soil, and then drawing off the contaminant in the vapor phase. Thus, conventional vacuum extraction systems are designed to clean the vadose zone by applying vacuum to draw air through the soil through wells having screening which does not extend below the water table. Ground water requiring treatment is in such processes conventionally removed by pumping from separate conventional water wells. In situations in which water does flow into vacuum extraction wells, it has been suggested that a second, liquid phase pump be placed either in the well or at the surface to remove the water through a second conduit. Thus, conventionally, water wells separate and apart from vacuum extraction wells may be required at a given site, and water pumps in addition to vacuum generation devices may be employed. Such extraction systems are therefore referred to as single phase extraction systems since contaminants are extracted in separate, single phases.
Processes and apparatus for two phase extraction of contaminants from the soil, in which contaminants are typically present in the vadose zone and below the water table have also been proposed. In accordance with the invention disclosed in U.S. patent application Ser. No. 413,237, filed on Sept. 27, 1989, a two phase vacuum extraction system, wherein a single vacuum device removes contaminants in both the liquid and vapor phase by way of a single conduit formed by the well casing can effectively remediate a site. The process disclosed involves the steps of providing a borehole in the contaminated area; placing a riser pipe in the borehole, the riser pipe preferably being constructed so as to admit fluids both from the vadose zone and from below the natural water table; applying a vacuum to the riser pipe so as to draw soil gases and entrained liquid into the riser pipe and to transport both the gases and the liquid to the surface; separating the liquid and the gases, and separately subjecting the separated liquid and gases to appropriate treatment. Treated water may be returned to the soil or disposed of in conventional ways. In one embodiment of this invention, the well casing is constructed with perforations (screening) extending below the natural water table and also upward into the unsaturated (vadose) zone. The unsaturated zone may be the natural vadose zone lying above the natural water table, or an expanded "artificial" vadose zone created when removal of the ground water through the extraction well causes local lowering of the water table. Placing of the screening so that it extends into the vadose zone allows soil gases, including contaminants in the vapor phase, to be drawn into the well under the influence of a vacuum generator. The gases, it has been found, entrain the liquid phase, so that both phases may be transported to the surface together in a common stream. At the surface, the two phases are separated in a vapor-liquid disengaging vessel, such as a cyclone separator, knock-out pot or other suitable component, and after separation the phases may individually be routed to systems for contaminant removal by further treatment steps. Suitable processes for contaminant removal include filtration, adsorption, air stripping, settling, flocculation, precipitation, scrubbing and the like.
As an alternative, to the above described process, U.S. patent application Ser. No. 413,273, filed on Sept. 27, 1989 also discloses a treatment well constructed so that screening is at all times is below the water table, even in the situation in which removal of water causes local depression of the water table. In such an arrangement, the fluid transported to the surface would predominantly be in the liquid phase, although it may still be necessary to provide vapor-liquid separation and individual phase treatment at the surface to deal with phase transformation which may occur as a result of turbulence and pressure reduction at the suction side of the vacuum device.
Two phase vacuum extraction in accordance with U.S. patent application Ser. No. 413,273, filed on Sept. 27, 1989 is an improvement over known soil and ground water remediation vacuum extraction techniques which simplifies equipment requirements and increases the rate of recovery of ground water. Unlike the prior art, water wells and pumps distinct from the extraction well are not required. A single vacuum device serves to remove contaminants in both the vapor and liquid phases, using a single conduit. However, a severe practical limitation to the system disclosed by U.S. patent application Ser. No. 413,273, filed on Sept. 27, 1989 exists in that, at best, a conventional vacuum pump can create a negative pressure of about one atmosphere (760 torr). Relating this fact to the hydrodynamic head of the well, one of ordinary skill will realize that when the riser pipe extends into liquid and the length of the riser pipe exceeds about thirty (30) feet, the vacuum pump will not withdraw soil gases and entrained liquid, since it will be unable to lift a column of liquid above to the soil surface. Therefore, at present, two phase extraction cannot be undertaken at sites where the surface of the water or liquid in the borehole is greater than about thirty feet beneath the soil surface. For similar reasons, in certain single phase extraction systems, the presence of a liquid layer makes the use of a single vacuum extraction system highly impractical.